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Mixed acoustic–entropy combustion instabilities in gas turbines

Motheau, Emmanuel and Nicoud, Franck and Poinsot, Thierry Mixed acoustic–entropy combustion instabilities in gas turbines. (2014) Journal of Fluid Mechanics, 749. 542-576. ISSN 0022-1120

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Official URL: http://dx.doi.org/10.1017/jfm.2014.245

Abstract

A combustion instability in a combustor terminated by a nozzle is analysed and modelled based on a low-order Helmholtz solver. A large eddy simulation (LES) of the corresponding turbulent, compressible and reacting flow is first performed and analysed based on dynamic mode decomposition (DMD). The mode with the highest amplitude shares the same frequency of oscillation as the experiment (approximately 320 Hz) and shows the presence of large entropy spots generated within the combustion chamber and convected down to the exit nozzle. The lowest purely acoustic mode being in the range 700–750 Hz, it is postulated that the instability observed around 320 Hz stems from a mixed entropy–acoustic mode, where the acoustic generation associated with entropy spots being convected throughout the choked nozzle plays a key role. The DMD analysis allows one to extract from the LES results a low-order model that confirms that the mechanism of the low-frequency combustion instability indeed involves both acoustic and convected entropy waves. The delayed entropy coupled boundary condition (DECBC) (Motheau, Selle & Nicoud, J. Sound Vib., vol. 333, 2014, pp. 246–262) is implemented into a numerical Helmholtz solver where the baseline flow is assumed at rest. When fed with appropriate transfer functions to model the entropy generation and convection from the flame to the exit, the Helmholtz/DECBC solver predicts the presence of an unstable mode around 320 Hz, in agreement with both LES and experiments.

Item Type:Article
Additional Information:Thanks to Cambridge University Press (CUP) editor. The original PDF of the article can be found at Journal of Fluid Mechanics website : http://journals.cambridge.org/action/displayJournal?jid=FLM
HAL Id:hal-01115554
Audience (journal):International peer-reviewed journal
Uncontrolled Keywords:
Institution:French research institutions > Centre National de la Recherche Scientifique - CNRS (FRANCE)
Université de Toulouse > Institut National Polytechnique de Toulouse - INPT (FRANCE)
Université de Toulouse > Université Toulouse III - Paul Sabatier - UPS (FRANCE)
Other partners > Université de Montpellier 2 (FRANCE)
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Deposited By: Thierry POINSOT
Deposited On:11 Feb 2015 11:20

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